The present invention relates generally to a method of geophysical exploration and more particularly to novel methods for acquiring and processing multicomponent seismic data obtained with seismic sources generating shear waves having time-varying polarizations.
Exploration geophysicists have generally considered the earth's subsurface formations to be locally homogeneous and isotropic. However, exploration geophysicists have recently discovered that the assumptions about the general homogeneity and isotropy of the earth's subsurface formations are incorrect and that the earth's subsurface formations can exhibit pronounced anisotropy, particularly azimuthal anisotropy. Alford et al. in U.S. Pat. No. 4,817,061, Alford in U.S. Pat. No. 4,803,666, and Thomsen in U.S. Pat. No. 4,888,743, describe geophysical methods for acquiring and processing multicomponent seismic data to ameliorate the deleterious effects of formation anisotropy on seismic exploration.
Many seismic sources, such as described by Willis et al. in U.S. Pat. No. 4,842,094, are considered as having fixed polarizations when they impart seismic energy into the earth, although they have the capability of imparting shear waves polarized along any selected polarization. With the advent of torsional seismic sources generating elliptically polarized shear waves as described by Cole in U.S. Pat. Nos. 4,867,096 and 4,871,045, we have discovered a novel method of geophysical exploration which can have a substantial impact on the acquisition and processing of multicomponent seismic data, especially in areas where the earth's subsurface formations exhibit azimuthally anisotropic characteristics. More particularly, the multicomponent seismic acquisition methods can be simplified. Consequently, the time required to collect multicomponent seismic data can be greatly reduced. These and other benefits of the present invention will be further described in more detail below.